13). S i m i l a r a n a l y s i s of a l l the responses l e d t o Table IV, wh'ich l i s t s a l l the c o e f f i c i e n t s and adjusted R f o r these responses. Apparently, not a l l the data are adequately described by the mathematical models, as shown by the low R for some. This point w i l l be discussed below. One of the main reason to use the models f o r p r e d i c t i n design space not a c t u a l l y covered by experiment. Such p r e d i c t i o n can be done mathematically, but i t i s u s u a l l y easier for the experimenter to look at graphical representations of the data.